This invention relates generally to the laser art and more particular to lasers which utilize upconversion-type materials as the lasing medium.
It is well known that some rare earth ions, when incorporated as impurities in sufficient concentration into a suitable host lattice, can upconvert infrared radiation to various shorter wavelengths. Upconversion, i.e., the conversion of long-wave into short-wave radiation by certain solids, without assistance of auxiliary radiation, can be accomplished by several multiphoton mechanisms. However, only one, the cooperative excitation mechanism, is thought to be efficient enough to be practical. The latter mechanism proceeds according to a scheme in which the ground-state electrons of several atoms (ions) absorb one infrared photon each. The energy subsequently migrates through a nonradiative process to a single atom exciting it to a higher energy level with ensuing fluorescence.
It has been proposed some time ago to utilize this mechanism for optical laser pumping; "Infrared-Pumped Visible Laser", L. F. Johnson, H. J. Guggenheim, Appl. Phys. Lett., Vol. 19, No. 2, pg. 44, 1971. Using conventional optical pumping, the population inversion of a lasing state is achieved through absorption of radiation by the laser active medium into energy states lying at or above the meta-stable lasing state. Subsequently, the energy is transferred to a lasing state (if previously above) via radiative and/or nonradiative transitions. In contrast, the upconversion optical pumping scheme is based on up-conversion of radiation absorbed into an energy level lying below the initial lasing state.
Pumping of a solid state infrared laser with upconverted vibrational energy has also been reported; "Solid-State Vibrational Laser - KBr:CN.sup.- ", R. W. Tkatch, T. R. Gosnell, A. J. Sievers, Optics Letters, Vol. 10, No. 4, pg. 122, 1984.